Fill tuple before HeapSatisfiesHOTAndKeyUpdate

Failing to do this results in almost all updates to system catalogs
being non-HOT updates, because the OID column would differ (not having
been set for the new tuple), which is an indexed column.

While at it, make sure to set the tableoid early in both old and new
tuples as well.  This isn't of much consequence, since that column is
seldom (never?) indexed.

Report and patch from Andres Freund.
Fix typo in freeze_table_age implementation

The original code used freeze_min_age instead of freeze_table_age.  The
main consequence of this mistake is that lowering freeze_min_age would
cause full-table scans to occur much more frequently, which causes
serious issues because the number of writes required is much larger.
That feature (freeze_min_age) is supposed to affect only how soon tuples
are frozen; some pages should still be skipped due to the visibility
map.

Backpatch to 8.4, where the freeze_table_age feature was introduced.

Report and patch from Andres Freund
Move Assert() definitions to c.h

This way, they can be used by frontend and backend code.  We already
supported that, but doing it this way allows us to mix true frontend
files with backend files compiled in frontend environment.

Author: Andres Freund
doc: Tiny whitespace fix